Magnet wires are used in the electromagnet windings found in electric motors, transformers, and other components having a conductive wire core. Such magnet wires are advantageously coated with various insulating or dielectric materials, including polymeric resins. Conventional polyamideimide enamels have been used as magnet wire coatings and in electronics packaging applications owning to their excellent thermal, mechanical, and electrical properties. Further, magnet wires may be precoated with a modified polyester and then over-coated with a polyamideimide. It has been shown that the polyamideimide overcoat provides improved mechanical properties over wires coated only with a polyester, including improved scrape resistance. Conventional polyamideimide enamels may be prepared using various copolycondensation reactions, such as the Carothers reaction of aromatic diisocyanates with a mixture of dicarboxylic acids and anhydrides of dicarboxylic acids to form an enamel solution as shown in Scheme 1:

where A is the residue of an anhydride moiety and optionally may include additional anhydrides or other substituents having active hydrogens, B is the residue of a diacid moiety, and C is the residue of a diisocyanate moiety. It is appreciated that the diacid HO2C—“B”—CO2H illustrated in Scheme 1 may also be the corresponding anhydride as shown. After filtration, such enamel solutions can be coated directly on magnet wires, or on magnet wires already having base coat, or other conventional pretreatment. These coated wires are cured in high temperature ovens, and a multiple pass process may be used for adequate curing and to provide an insulation protective layer having the surface smoothness necessary for coated wire applications. Conventional polyamideimide enamels prepared from such enamel solutions generally possess a linear chain backbone having both imide and amide linkages. These amide and imide groups may account for the observed thermal, mechanical, and electrical properties of these polymers.
The mechanical properties of the polyamideimide coatings can be evaluated using standard tests. For example, 18-gauge copper wire over-coated with conventional polyamideimide coatings generally shows a film hardness and NEMA unidirectional scrape resistance of about 1350-1450 grams, and repeated scrape resistance of about 50-100 strokes. Higher weight values and higher numbers of strokes, respectively, correspond to better scrape resistance. Although polyamideimide overcoated wires readily meet NEMA standard tests, recent advancements and modifications in motor manufacturing processes, such as fast coil winding, tough coil insertion, and environmental friendly varnishing processes, have generated the need for wire coatings that meet more stringent mechanical property requirements, such as increased film hardness and increased scrape resistance. It is further desirable that wire coatings maintain minimal electrical and thermal properties for each applied use, and all at comparable or lower overall costs with fewer negative consequences for the natural environment.
To meet these new demands, coatings that have properties such as improved unidirectional scrape resistance and/or improved repeated scrape resistance are desired. One way to provide such improved mechanical and thermal properties is by modifying the overcoat, such as a modified polyamideimide over-coat. Such modification may include changes in the chemical structure of the polymer molecules, or changes to the morphological structure of the aggregate of polymer molecules forming the overcoat.
Current methods for improving such mechanical properties generally involve increasing the molecular weight of the polyamideimide, increasing the aromatic character of or the number of aromatic moieties on the chain backbone, or increasing the oven curing time of the enamel. However, even slight increases in the molecular weight of these linear polymers may result in a concomitant and often drastic increase in the viscosity of the coating composition, because the viscosity of the coating solution follows the conventional viscosity laws and is therefore a function of the molecular weight of the viscoelastic polymer. Thus, in order to keep the viscosity within acceptable ranges, the solids content of the enamel solution is typically lowered to compensate. This lowering of solids content may result in increased processing and manufacturing costs and increased waste disposal costs.
Further, the strategy of increasing the aromatic character of or the number of aromatic moieties on the chain backbone also may lower the solubility of the polyamideimide, often resulting in the need to reduce the solids content of the enamel with the accompanying economic disadvantages described above. Finally, increasing the enamel curing time tends to increase overall production costs, and may result in a decrease in film flexibility, or result in dark-cured films.
Other efforts in improving polyamideimide coating compositions are described in French Patent No. 00408473 (disclosing that linear, aromatic polyamideimide can be functionalized through terminal groups containing latent maleimide functions for crosslinking); Kim Hwan Kyu et al., in Polymer, Vol. 39, No. 8-9, p 1719-26, 1998 (disclosing that branching stilbene derivatives can be attached onto a polyamideimide chain for second-order non-linear optics application); Japanese Patent No. 08165347 (disclosing that a polyamideimide chain containing a linear or branched C2-C20 alkylene chain for adhesive and coating applications); U.S. Pat. No. 6,319,604 (disclosing an abrasion resistant coated wire prepared using polyamideimide/nitride ceramic coatings).